I'm on the same boat as everybody else: doubles only if they're from distinctly different canons/alternate universes (i.e. Kingdom Hearts vs Final Fantasy, etc). Definite no to doubles of characters who are simply from different points in the same timeline. (As much fun as it would be to see a Crisis Core Sephiroth pop in for the lulz...) Doubles occurring in canon are okay too. Muns of double characters will probably want to discuss any issues that might arise, though that'll be their responsibility.

So yeah. Another vote for AU/canon-occurring doubles.